Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
2001-10-12
2004-09-21
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C356S440000
Reexamination Certificate
active
06795190
ABSTRACT:
TECHNICAL FIELD
The present invention relates to absorption spectroscopy methods and apparatus, and in particular to those methods and apparatus which employ an optical cavity for increasing detection sensitivity, especially ones adapted for cavity ring-down spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS). Arrangements, active or passive, for reducing sensitivity of the instrument to alignment or vibrations, including those that manipulate or control optical resonances of the instrument's cavity are particularly relevant.
BACKGROUND ART
Cavity ringdown spectroscopy (CRDS) and integrated cavity output spectroscopy (ICOS) methods and associated instruments employ optical cavities (also known as “etalons”) as absorption cells for spectroscopic purposes. These spectroscopy methods and instruments have a broad range of other applications, such as characterizing mirror reflectivities, determining optical cavity losses (including scattering, absorption, etc.) and measuring thin film absorption. Other potential uses include using the invention for quantitative chemical analysis systems for applications such as offgas monitoring, medical diagnostics (such as breath analysis), trace gas analysis, thin film analysis, pollution monitoring, process control monitoring, purity analysis, and toxic chemicals detection. Although the background discussion will focus on the specific development of absorption techniques for gas phase chemical detection and characterizing optical components such as high reflectivity, low loss mirrors, it is by no means limited to this application.
The state of the art in spectrophotometer technology and spectroscopic techniques used for the purpose of spectrally characterizing solids, liquids, and gases includes absorption, emission, and ionization-based techniques. Recent developments in optical components and spectrally bright light sources have led to a variety of spectroscopic-based techniques and instruments that provide chemical analysis data of gaseous samples. Conventional Absorption (CA) or Emission (EM) spectroscopies are currently implemented for the quantitative analysis of chemical species in gases as a means of providing concentration information, but these methods frequently require tedious calibration procedures (EM) or suffer from low sensitivity. In the case of emission spectroscopy, inter and intramolecular dynamics, such as internal conversion or predissociation, can significantly degrade the ability to both detect and quantify species concentrations. Although much less effected by these dynamical processes, CA historically suffers from lower sensitivity (compared to that of EM, for example), and hence cannot typically achieve similar detection sensitivities. Another technology currently in use for chemical monitoring is Laser Spark Spectroscopy (LS), which involves measuring emission spectra of species that are vaporized with an intense laser pulse. Although highly sensitive, LS is only capable of identifying the presence of metals, and is not generally capable of providing absolute concentrations for those species without elaborate calibration procedures. A primary reason for employing CA is the relative ease with which absolute species concentration can be determined from the associated absorption spectra. To circumvent the historically lower sensitivity of absorption spectroscopy, several methods have been developed. Absorption-based optical detection methods which enable chemical concentrations to be determined include frequency (or amplitude) modulated laser absorption spectroscopy (FM-LAS) and Fourier transform spectroscopy (FTS). Although these methods have been used with some success, they can suffer from low sensitivity (FTS), or can be difficult or impossible to implement in spectrally congested areas and have limited spectral coverage (FM-LAS).
An alternative to these methods involves the use of high finesse optical cavities, which have long been known to amplify optical loss processes occurring between the cavity optics. (Jackson, D. A.,
The Spherical Fabry
-
Perot Interferometer as an Instrument of High Resolving Power for use with External or with Internal Atomic Beams
. Proc. R. Soc. London Ser. A, 1961. 263: p. 289.) Ultimately, this allows highly sensitive measurements of such processes as molecular absorption to be achieved. Several methods have been described to use optical cavities for such purposes. (Scherer, J. J., et al.,
Cavity ringdown laser absorption spectroscopy—history, development, and application to pulsed molecular beams
. Chemical Reviews, 1997 January-February. 97(1): p. 25-51. Romanini, D., A. A. Kachanov, and F. Stoeckel,
Diode laser cavity ring down spectroscopy
. Chemical Physics Letters, 30 May 1997. 270(5-6): p. 538-45. Ye, J., L.-S. Ma, and J. L. Hall,
Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy
. J. Opt. Soc. Am. B, 1998. 15(1): p. 6. Paldus, B. A., et al.,
Cavity
-
locked ring
-
down spectroscopy
. Journal of Applied Physics, 15 Apr. 1998. 83(8): p. 3991-7.) One of the most common, known as cavity ringdown spectroscopy (CRDS), measures the total optical cavity loss by monitoring the decay of the intracavity intensity following the injection of radiation into the cavity, either by a single laser pulse, (O'Keefe, A. and D. A. G. Deacon,
Cavity ring
-
down optical spectrometer for absorption measurements using pulsed laser sources
. Review of Scientific Instruments, December 1988. 59(12): p. 2544-51.) or by an abruptly interrupted continuous-wave (CW) laser. (Anderson, D. Z., J. C. Frisch, and C. S. Masser,
Optical reflectometer based on optical decay time
. Appl. Opt., 1984. 23: p. 1238.) The ringdown process itself was patented by Litton Corporation (U.S. Pat. No. 4,793,709, Method and apparatus for measuring losses of an optical cavity, issued Dec. 27, 1988, now expired.) for the specific purpose of determining mirror reflectivities. Since this patent, CRDS has been developed (and extensively published in the open literature) for the specific purpose of determining atomic and molecular absorption for species located within the optical cavity (between the mirrors). The currently well established and practiced pulsed CRDS method was first developed by O'Keefe and Deacon in 1988, who demonstrated its high sensitivity and spectroscopic capabilities by measuring weak visible absorption by molecular oxygen. (O'Keefe, A. and D. A. G. Deacon,
Cavity ring
-
down optical spectrometer for absorption measurements using pulsed laser sources
. Review of Scientific Instruments, December 1988. 59(12): p. 2544-51.) Additionally, continuous-wave versions of CRDS that are based on the original, early versions of the technology have been developed for the specific task of obtaining absorption spectra of chemical species placed in the cavity. (U.S. Pat. No. 5,528,040
, Ring
-
down cavity spectroscopy cell using continuous wave excitation for trace species detection
, Jun. 18, 1996.) Other cavity-based methods, such as integrated cavity output spectroscopy (ICOS) (O'Keefe, A., J. J. Scherer, and J. B. Paul,
CW integrated cavity output spectroscopy
. Chemical Physics Letters, 9 Jul. 1999. 307(5-6): p. 343-9.) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) (Ye, J., L.-S. Ma, and J. L. Hall,
Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy
. J. Opt. Soc. Am. B, 1998. 15(1): p. 6.) use the cavity transmission properties to gauge the intracavity loss, but in these cases the intrinsic cavity loss must be determined separately, by using CRDS for example, to obtain quantitative absorption intensity data.
Many of the above methods, in particular those employing narrowband CW laser sources, are designed to manipulate or control the optical resonances that arise within cavity due to the periodic boundary conditions imposed on the intracavity electric field by the mirror surfaces. These resonances, which are interferome
O'Keefe Anthony
Paul Joshua B
Scherer James J
Font Frank G.
Los Gatos Research, Inc.
Merlino Amanda
Protsik Mark
Schneck Thomas
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